Thermally induced shape memory polymer can fix a temporary deformed shape and stay in that shape stably when the temperature is below its transition temperature. On the other hand, the polymer will return to its original shape when the temperature rises higher than the transition temperature. In the molecular level, thermally induced shape memory polymer should have a reversible switch and the netpoints. There are two types of reversible switch, namely amorphous segment and semi-crystalline segment. The transition temperature of the corresponding amorphous polymer and semi-crystalline polymer are glass transition temperature and melting temperature respectively. The netpoints can be physical ones such as hard segment reinforcement, molecular entanglement in linear structural polymer or chemical ones in covalently cross-linked polymer. A great number of amorphous and semi-crystalline shape memory polymers can be designed, for example in U.S. Pat. No. 6,160,084, U.S. Pat. No. 6,720,402B2, U.S. Pat. No. 7,935,131B2, U.S. Pat. No. 8,172,873B2 and U.S. Pat. No. 6,388,043B1.
In some scenarios, it is desirable to design a shape memory polymer with a transition temperature near body temperature (28° C. to 40° C.). For instance, a garment, in close contact with human body with a transition temperature near body temperature, can fit the body shape very well and do not lose retention power while wearing. Another example would be the use of a shape memory polymer as an implantation material in human body. In such cases, during the implantation process of an implantation material having a transition temperature near body temperature, no additional heating process is required, which is usually either complicated or dangerous since the heat may be detrimental to the human body.
Among different shape memory polymer candidates, polyurethane is one of the most versatile materials. Its chemical structure and physical properties are highly adjustable to meet various applications with easy processing method and low production cost. Shape memory polyurethane as a smart material is developed and researched since 1980s. For example, U.S. Pat. No. 6,583,194B2 and U.S. Pat. No. 5,049,591 disclosed shape memory polyurethane foams with glass transition temperature at or above room temperature. U.S. Pat. No. 5,155,199 disclosed shape memory polyurethane fine particles used in makeup materials. U.S. Pat. No. 5,098,776 disclosed a method to fabricate shape memory fibrous sheet. U.S. Pat. No. 5,145,935 disclosed shape memory polyurethane elastomer molded article. U.S. Pat. No. 5,135,786 disclosed shape memory polyurethane transparent body. U.S. Pat. No. 5,128,197 disclosed woven fabric made of shape memory polymer. U.S. Pat. No. 6,858,680B2 disclosed a shape memory polyurethane or polyurethane-urea polymers. Nonetheless, previous research focused in developing amorphous shape memory polyurethane with glass transition temperature as the transition temperature, in which the glass transition temperature is tunable to body temperature. However, semi-crystalline shape memory polyurethane with melting temperature around body temperature has never been disclosed. It has been suggested that the melting temperature of semi-crystalline shape memory polyurethane is about 10° C. lower than that of the polyester diol which is used as the starting material. In order to adjust the melting temperature of the semi-crystalline shape memory polyurethane to near body temperature, the melting temperature of the corresponding polyester diol should be around 30° C. to 50° C. However, the melting temperature of most polyester diols is around 50° C. to 60° C. while the molecular weight is above 2000 g/mol. Although the melting temperature of polyester diol can be decreased to below 50° C. by reducing its molecular weight to below 2000 g/mol, the resulting polyurethane does not exhibit shape memory effects. This may be due to the fact that the hard segment, especially the rigid moieties in the diisocyanate, impedes the crystallization of the soft segment of the polyurethane when low molecular weight polyester diol is used.
Although no prior patents reveal semi-crystalline shape memory polyurethane with transition temperature near body temperature, there are some journal publications describing such shape memory polymer. For example, in the paper [Macromolecules 2009, 42, 964-972], a crosslinking shape memory polyurethane using three-arm polycaprolactone triol as the soft segment is developed to possess melting temperature near body temperature. The three-arm polycaprolactone triol is synthesized using a special catalyst. The body temperature sensitive shape memory polyurethane developed has shown the huge potential in application. However, the complicated synthesis process and the crosslinking structure obstruct such smart material for the industrial mass production with acceptable manufacturing cost.